Flavour aldehydes

Scheme 23.2 Production of aliphatic flavour aldehydes from natural alcohols using alcohol oxidase activity of Pichia pastoris cells...

Scheme 233 Formation of aliphatic flavour aldehydes and alcohols, a Biotechnological reaction sequence mimicking plant biosynthesis of C6 compounds (green notes ), b HomologoiK reaction sequence in fimgi leading to mushroom-like C8 compounds. The stoichiometric formation of w-oxo-carboxylic acids during hydroperoxide lyase cleavage is not depicted...

The potential applications of oxidoreductases in industry are numerous. Alcohol dehydrogenases are useful in stereospecific synthesis and production of high-cost compounds such as cyclic ethers (see below), and have also been used in the production of flavour aldehydes e.g. geraniol [94]. With mesophilic enzymes, however, limitations due to narrow specificity, instability to heat and organic solvents and loss of activity on immobilization have been incurred. The use of these enzymes from thermophilic sources in inunobilized and continuous reactor systems (including biosensors) has also been proposed for the regeneration of... 

The former is the more probable constitution. Citral as found in commerce, is probably almost invariably a mixture of the two isomers, which are very similar in their general characters. It occurs to a considerable extent in various essential oils, being the principal constituent of lemon-grass oil, and of the oil of Backhmisia citriodora, and occurring to the extent of about 4 to (i irer cent, in lemon oil, which owes its characteristic flavour to this aldehyde. 

Anisic aldehyde Pharma Aroma and Flavour Agrochemical C (Homo Het) 1000 - 3000 STR EC... 

No attempt is made to provide comprehensive coverage of all the work carried out in these different media, but rather to give a flavour of the kind of systems for which the different approaches may be appropriate. In all the chapters, a more detailed discussion of the rhodium catalysed hydroformylation of 1-octene to nonanal, as a representative example of the synthesis of a long chain aldehyde with relatively low volatility, is provided [13, 14], This reaction has been chosen because ... 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

Benzaldehyde is an important intermediate for dyes (see malachite green) many other aldehydes (phenylacetaldehyde, vanillin, piperonal, citral, etc.) are used in perfumery or as flavouring agents. 

Aldehydes, ketones and carboxylic acids are widespread in plants and animal kingdom. They play an important role in biochemical processes of life. They add fragrance and flavour to nature, for example, vanillin (from vanilla beans), salicylaldehyde (from meadow sweet) and cinnamaldehyde (from cinnamon) have veiy pleasant fragrances. 

The MHLW is currently evaluating several individual flavouring substances not covered by the aforementioned groups but that are of commercial interest. Most of these substances are lower alcohols, aldehydes and pyrazines. As soon as the evaluation has finished, the result will be published and in positive cases the substances will by added to the list of permitted substances. 

Orange peel oil is the major oil produced worldwide and is used extensively in the food industry, primarily as a flavouring in beverages and sweets. It possesses a light, sweet, fresh top note with fruity and aldehydic character. Many household and personal-care products employ orange oil owing to its pleasing... 

Fig. 7.3 Some aliphatic alcohols, aldehydes and ketones which are important flavour compounds in fruits and vegetables that mainly contribute with green and/or sweet notes...

The berry or the small fruits consist of strawberry, raspberry, blackberry, black currant, blueberry, cranberry and elderberry. The volatiles responsible for the flavour of small fruits are esters, alcohols, ketones, aldehydes, terpenoids, furanones and sulfur compounds (Table 7.3, Figs. 7.1-7.7). As fruit ripen, the concentration of aroma volatiles rapidly increases, closely following pigment formation [43]. 

Sugars, acids and aroma compounds contribute to the characteristic strawberry flavour [85]. Over 360 different volatile compounds have been identified in strawberry fruit [35]. Strawberry aroma is composed predominately of esters (25-90% of the total volatile mass in ripe strawberry fruit) with alcohols, ketones, lactones and aldehydes being present in smaller quantities [85]. Esters provide a fruity and floral characteristic to the aroma [35,86], but aldehydes and furanones also contribute to the strawberry aroma [85, 87]. Terpenoids and sulfur compounds may also have a significant impact on the characteristic strawberry fruit aroma although they normally only make up a small portion of the strawberry volatile compounds [88, 89]. Sulfur compounds, e.g. methanethiol. 

The fruity-sweet flavours in elderberry juice and products have primarily been associated with aliphatic esters such as ethyl 2-methylbutanoate, ethyl 3-methylbutanoate, methyl heptanoate, methyl octanoate, methyl nonanoate, alcohols (2-methyl-l-propanol, 2-methyl-1-butanol and 3-methyl-1-butanol) and the aldehydes pentanal, heptanal and octanal [127, 129, 130, 132],... 

The roundish flower head, the curd, of the cauliflower plant (Brasska oleracea var. botrytis) is the edible portion of this vegetable. It can be eaten raw in salads or as a pickled condiment in vinegar. More often it is boiled and eaten with the main meal or is converted into sauces and soups. Over 80 volatile compounds have been identified in raw and cooked cauliflower. Among the compounds potentially active in cooked cauliflower, certain sulfides such as methanethiol, dimethyl sulfide and dimethyl trisulflde have often been incriminated in objectionable sulfurous aromas and overcooked off-flavours [169, 177, 178, 181-183]. Additional aldehydes have been found to be the most abundant cauliflower volatiles, with nonanal as a major component [175,177]. A recent study showed that volatiles such as 2-propenyl isothiocyanate, dimethyl trisulflde, di-... 

Essences of pink and white fresh guava obtained by direct extraction of flesh juices with dichloromethane revealed that the total amount of Cs aldehydes, alcohols, and acids comprised 20 and 44% of the essence of fresh white and pink guavas, respectively [49]. The flavour of the Costa Rican guava has been described as sweet with strong fruity, woody-spicy, and floral notes [53]. One hundred and seventy-three volatile compounds were isolated by simultaneous steam distillation-solvent extraction. The terpenes and terpenic derivatives were found in this fruit in major concentrations and were strong contributors to tropical fruit notes (Fig. 8.1). The aliphatic esters contributed much to its typical flavour. 

Charentais cantaloupe melon Cucumismelo L. var. cantalupensis Naud.) was characterised by abundant sweetness and a good aromatic flavour [68]. The aroma volatiles of Charentais-type cantaloupe melons, as with other cantaloupes, comprise a complex mixture of compounds including esters, saturated and unsaturated aldehydes and alcohols, as well as sulfur compounds [26, 65]. Among these compounds, volatile esters were quantitatively the most important and therefore represent key contributors to the aroma [68]. The linear saturated and unsaturated aldehydes seem to originate from the degradation of linolenic and linoleic acids [26, 32, 33, 67]. 

The first group contains compounds produced in the early stages of the reaction by the breakdown of the Amadori or Heynes intermediates, and includes similar compounds to those found in the caramelisation of sugars. Many of these compounds possess aromas that could contribute to food flavour, but they are also important intermediates for other compounds. The second group comprises simple aldehydes, hydrogen sulphide or amino compounds that result from the Strecker degradation occurring between amino acids and dicarbonyl compounds. 

Cinnamon oil trans-Cinnamic aldehyde Flavours for confectionery products... 

There are synthetic acetal derivatives of flavour-active aldehydes like benzaldehyde and cinnamaldehyde [59]. 

Fig. 23.1 Microbial routes from natural raw materials to and between natural flavour compounds (solid arrows). Natural raw materials are depicted within the ellipse. Raw material fractions are derived from their natural sources by conventional means, such as extraction and hydrolysis (dotted arrows). De novo indicates flavour compounds which arise from microbial cultures by de novo biosynthesis (e.g. on glucose or other carbon sources) and not by biotransformation of an externally added precursor. It should be noted that there are many more flavour compounds accessible by biocatalysis using free enzymes which are not described in this chapter, especially flavour esters by esterification of natural alcohols (e.g. aliphatic or terpene alcohols) with natural acids by free lipases. For the sake of completeness, the C6 aldehydes are also shown although only the formation of the corresponding alcohols involves microbial cells as catalysts. The list of flavour compounds shown is not intended to be all-embracing but focuses on the examples discussed in this chapter... 

Short-chain aliphatic aldehydes, such as acetaldehyde, 2-methyl-1-propa-nal, 2-methylbutanal and 3-methylbutanal (isovaleraldehyde), impart fruity and roast characters to flavour compositions [49]. Natural acetaldehyde is an important compound naturally occurring in a broad range of fruit flavours, essential oils and distillates it augments fruit flavours and, for instance, it decisively contributes to the freshness and juiciness of foods and beverages, such as citrus juices [23, 50]. 

Nevertheless, for the production of the flavour-active aromatic alcohol derivatives, such as the corresponding aldehydes and acids, metabolic engineering approaches have to compete with conventional oxidative biocatalysis starting from the natural alcohol as a substrate. For instance, the whole-cell oxidation system based on Pichia pastor is AOX already described in Sect. 23.4.1.2 can also be used to convert benzyl alcohol to benzaldehyde in aqueous media although product inhibition restricted the final product concentration to about 5 g L h indicating the need for aqueous-organic two-phase reaction media [51]. Phenylacetalde-... 

Since cinnamyl aldehyde is the main component of cassia oil (approximately 90%) and Sri Lanka cinnamon bark oil (approximately 75%) [49], it is industrially more important to generate cinnamyl alcohol, which is less abundantly available from nature but is important as cinnamon flavour, by biotransformation of natural cinnamyl aldehyde than vice versa. Recently, a whole-cell reduction of cinnamyl aldehyde with a conversion yield of 98% at very high precursor concentrations of up to 166 g L was described [136]. Escherichia coli DSM 14459 expressing a NADPH-dependent R alcohol dehydrogenase from Lactobacillus kefir and a glucose dehydrogenase from Thermoplasma acidophilum for intracellular cofactor regeneration was applied as the biocatalyst (Scheme 23.8). 

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